Aluminum-anodizing process

ABSTRACT

This invention generally relates to the anodic oxidation of aluminum articles in an aqueous sulfuric acid electrolyte. More particularly, this invention is directed to improving the corrosion resistance of the clear, colorless anodic oxide coatings formed in an aqueous sulfuric acid anodizing process by utilizing an aqueous electrolyte containing 7 to 12 percent sulfuric acid and 0.25 to 0.80 percent chromic acid.

United States Patent Inventor Geoffrey Austin Dorsey, Jr.

Spokane, Wash.

Mar. 10, 1969 Oct. 26, 197 1 Kaiser Aluminum & Chemical Corporation Oakland, Calif.

App]. No. Filed Patented Assignee ALUMINUM-ANODIZING PROCESS 2 Claims, No Drawings U.S. Cl 204/58 Int. Cl C23b 9/02 Field of Search 204/58, 35

References Cited UNITED STATES PATENTS 4/1965 Hollingsworth 204/29 FOREIGN PATENTS 1/1935 France OTHER REFERENCES Surface Treatment of Aluminum, by Wernick and Pinner, 3rd edition, 1964, pp. 356- 7, 385.

Primary ExaminerJohn H. Mack Assistant Examiner-R. L. Andrews An0meysJames E. Toomey, Paul E. Calrow, Harold L.

Jenkins and Frank M. Hasen ALUMlNUM-ANODIZING PROCESS BACKGROUND OF THE INVENTION Anodizing an aluminum surface in a sulfuric acid electrolyte in a well-known process for improving the corrosion resistance of aluminum. Aqueous electrolytes from 1-70 percent sulfuric acid concentrations can be used, although the preferable concentration is maintained between l and 25 percent. About 15 percent is presently the most frequently used level of sulfuric acid concentration for clear, colorless anodic oxide coatings. All percentage concentrations disclosed herein are percent by weight.

In the anodizing process utilizing a 15 percent sulfuric acid electrolyte the current density is normally maintained at about 12 a./s.f. and the temperature at about 25 C. A more protective coating can be obtained by lowering the temperature of the bath and/or by increasing the current density, but the image clarity of the resulting oxide coating is very poor. (It should be noted that all circuits described here in the specification and the appended claims are direct current or undulating direct current circuits unless stated otherwise). Other electrolytes such as aqueous solutions of chromic or oxalic acids produce more durable coatings; however, processes employing these electrolytes suffer from the same disadvantages, in that they do not produce clear, colorless anodic oxide coatings. Furthermore, they are much more expensive than the sulfuric acid processes due to the higher cost of chemicals and the higher power requirements for chromic and oxalic acid anodizing.

DESCRIPTION OF THE INVENTION The purpose of this invention is to provide a method of increasing the protective value or durability of anodic oxide coatings formed in an aqueous sulfuric acid electrolyte, without affecting the clarity or color (or rather, the lack thereof) of the oxide layer.

The present inventor has discovered that within certain narrow ranges of current density and electrolyte temperatures a small amount of chromic acid in an aqueous electrolyte containing 7 to 12 percent sulfuric acid will form anodic oxide coatings having improved durability to attacking environments, without detrimentally affecting the clarity or the color of the coating. It has been discovered that if the current density is maintained between 10 and 30 a./s.f. and the temperature of the electrolyte maintained between 20 and 30 C. small amounts of from 0.25 to 0.80 percent chromic acid in an aqueous electrolyte containing 7 to 12 percent sulfuric acid will form a more highly corrosionresistant anodic coating than that produced in the standard 15 percent sulfuric acid electrolyte.

Others, such as Speer, in U.S. Pat. No. 2,437,620 and Cohn in U.S. Pat. No. 2,578,400 have employed aqueous solutions of sulfuric acid and chromic acid as electrolytes for aluminum anodizing; however, neither recognized that improved durable coatings could be obtained within the rather narrow limits of sulfuric acid and chromic acid concentrations of the present invention, Furthermore, neither recognize that these small amounts of chromic acid added to the 7 to 12 percent sulfuric acid electrolytes would produce clear, colorless anodized coatings which are desirable in many architectural and automotive trim applications. Moreover, Speer added the chromic acid to the sulfuric acid electrolyte merely to prevent copper from plating out and in no way sought to improve the durability of the anodic oxide coating produced during anodization.

It is well known that most anodic oxide coatings on aluminum are characterized by two layersa barrier layer, lying next to the bare metal and a porous layer adjacent or on top of the barrier layer. The inventor has further discovered that this barrier layer is composed of two distinct phases or layersa primary layer or phase and a secondary layer or phase of various thickness, depending upon the electrolyte and anodizing conditions employed.

During the initial stages of anodization there is formed a primary phase which apparently is composed of cyclic alumina trihydrate. Soon thereafter, the porous layer begins to develop by the dehydration of the outer layers of the primary phase to form fibrils of polymeric aluminum oxide. Subsequently, the electrolyte begins attacking the outer surface of the primary phase, decyclizing and structurally rearranging the primary phase, thus forming the secondary phase. In most anodizing processes, the rate of formation of the secondary phase from the primary phase exceeds the rate of formation of the porous layer form the secondary phase and thus, as a result, most of the barrier layer is in the form of the secondary phase oxide. The cyclic alumina trihydrate in the primary phase appears to be hydrogen bonded (AlOH'OAl bonds) and is quite unreactive, possibly contributing considerably to the corrosion resistance of the anodic coating. The tightly bound trihydrate network is partly evidenced by the fact that the primary phase does not lose most of its combined waters of hydration unless heated above 400 C. As a contrast most mineralogical forms of the trihydrate lose their waters of hydration at about 325 C. The decyclized trihydrate in the secondary phase is charac- H lAl- 0 -Al) bonds and is also relatively unreactive. Due to the t iydafig effect of the electrolyte some Al=0 bonding is present in both the secondary phase of the barrier layer, and in the porous layer as well. This bonding is reactive and will be a potential site for coating attack in a corrosive environment. The polymeric aluminum oxide of the porous layer is characterized by both Al-O-Al and Al 0 type bonds, and, if this polymeric aluminum oxide is not completely dehydrated, there may be some mu-ltydroxy bonds. However, if the alumina fibrils are cross-linked, i.e., more highly polymerized, the Al 0 bonds are transformed to Al-O-Al linkages. This latter species might be expected to be quite unreactive by comparison.

The inventor has found that these various chemical bonds have characteristic infrared absorption wave number bands and that by determining the absorbance of these bands, the relative amounts of these chemical bonds can be determined for different anodic coatings. The wave number range for various bonds discussed above are shown in table I.

terized by both hydrogen and mu-hydroxy TABLE I Alumina-system chemical bond Infrared absorption Al=0(stretch) From I696 to 1345 The presence of double bonds indicates an adsorptive or reactive material that probably does not have much crosslinking.

Absorption here indicates the presence of aluminum hydroxides. Monohydrates are characterized by bands near 1070 cm' The primary phase cyclic trihydrate is characterized by bands near 950 cm".

AIOAI Bands in this region indicate a high (stretch) degree of crosslinking in the H AI material: strong bands in this 0 region are often associated with an lslrelch' absence of Al=0 bonds. As the degree of crosslinking. ie molecular weight. increases. the bond absorption frequency will shift to lower values.

Al-OH (bend) From I162 to 900 Below 900 cm Itshould be noted that with the Al-O-Al bonds the placement of the absorption bands depends on the molecular weight of the polymeric alumina, i.e., the more cross-linking the lower the wave number of the IR band. This degree of cross-linking, as will be discussed hereinafter, is thought to be a significant factor in governing the durability of an anodic coating. The relative degree of cross-linking can be conveniently expressed by the following formula:

Band absorbance Degree of cross-linking= Wavenumber of band peak The numerical value 900 is the maximum wave number limit for Al-O-Al and mu-hydroxy stretch vibrations.

Armed with the above information, the inventor examined the effects of various electrolytes on the relative amounts of the various types of bonds discussed above, in relation to their contribution to the corrosion resistance imparted to the aluminum substrate by the overlying anodic oxide layers. Chromic acid, oxalic acid, sulfuric acid and phosphoric acids were tested. From these investigations it was determined that of the various species of aluminum oxide present in the coating, the cyclized alumina trihydrate of the primary layer and the polymeric aluminum oxide of the porous layer are major factors in the corrosion resistance of the anodic coating.

While the Al bonds are quite reactive the improved durability of the anodic coating does not apparently result from their absence, i.e., removing sites for corrosion activity. The improved durability results instead from the increased alumina cross-linking within the porous layer. It was found that the anodic coating produced in the chromic acid electrolyte, which is known to be the most protective of those tested, has considerable amounts of the apparently cyclized trihydrate barrier layer and also has the most highly cross-linked porous layer. Phosphoric-acid-produced coatings, well known to af ford the least protective value of those tested, also has sizable amounts of this same type of barrier layer but practically no cross-linking of the alumina fibrils could be found.

Furthermore, it was found that when the anodic oxide coatings are sealed, part of the secondary phase is transformed to an alumina trihydrate having substantially the same structure as the hydrated porous layer. Indeed during sealing the secondary phase appears to partially transform into the hydrated porous phase. This is a surprising finding, for generally is was believed that sealing affected only the outer layers of the porous layer. Moreover, the hydration. and structural rearrangement of the secondary phase further increased the cross-linking of the porous layer. It is believed that this hydration and structural rearrangement, which is termed inner seal effect by the inventor, may contribute considerably to the durability of the anodic oxide coating. The magnitude of the change of the secondary phase to the porous layer trihydrate during scaling is controlled by the structure produced during anodization, which in turn is controlled by the electrolyte composition and the anodizing conditions. In the above-mentioned test work with various electrolytes it was found that the anodic oxide formed in chromic acid electrolytes had the greatest magnitude of inner seal effect. Neither the nature of the structure nor the relationship of structure to the inner seal effect is presently entirely understood.

The problem then arose as to how to increase the cross-linking in the porous layer of the anodic oxide coating produced in a sulfuric acid electrolyte without detrimentally affecting the clarity or color characteristics of the oxide coating.

The inventor has discovered that by adding controlled amounts of chromic acid to sulfuric acid electrolytes that the degree of cross-linking of the porous layer and the magnitude of the inner seal effect can be considerably increased. However, this higher degree of cross-linking, without affecting the clarity or color of the oxide coating, can be obtained only under very closely controlled process conditions. The sulfuric acid concentration should not exceed 12 percent or be lower than 7 percent. The chromic acid concentration should be maintained between 0.25 percent and 0.80 percent, that is between 2.5 and 8.0 grams/liter. The current density, although preferably kept at about 12 a./s.f., can range between and 30 a./s.f. The electrolyte temperature, preferably kept at about 25 C., can range between 20 and 30 C. For optimum corrosion resistance the aqueous electrolyte should contain about 0.5 percent chromic acid and about 10 percent sulfuric acid.

During anodization the chromic acid is reduced at the cathode and thus must be periodically replenished. it is preferred to start the anodization at a chromic acid concentration slightly above the optimum, about 0.65 percent, allowing the chromic acid concentrations to be reduced to about 0.3 percent and then replenishing the chromic acid to the 0.65 percent level. Obviously, if the rate of chromic acid reduction is known, the chromic acid may be added continuously. The cathodic reduction of the chromic acid can be substantially eliminated by utilizing an anodized aluminum cathode which prevents the diffusion of the dichromate ion to the metal substrate. The chromic acid may be added in the form of chromium oxide (CrO or chromic acid producing salts such as Na cr O. The chromic acid concentration is expressed herein as weight percent Cr03.

At the lower concentrations of sulfuric acid, higher concentrations of chromic acid can be tolerated and indeed there is some evidence that improved corrosion resistance can be obtained. However, at these higher concentrations of chromic acid, undesirable yellow hues develop during anodization and the image clarity is severely reduced. For example, a 7 percent sulfuric acid electrolyte can tolerate up to about 4 percent chromic acid, but the anodic coating could not be considered clear and colorless, and would be useless in automotive and architectural applications which demand these properties. At the higher concentrations of sulfuric acid the addition of small quantities of chromic acid do not appear to give a beneficial result. Indeed a 0.5 to 1 percent addition of chromic acid to a conventional 15 percent sulfuric acid electrolyte appears to slightly decrease the durability of the anodic oxide coating, rather than to increase it. For example, an anodic oxide coating produced in a 15 percent sulfuric acid electrolyte has a degree of cross-linking of 2.4; whereas, the anodic coating produced in such an electrolyte with about 1 percent chromic acid has a degree of cross-linking of about 2.2.

The following examples are given to further illustrate the features of the invention and are not intended to limit the scope of the present invention.

EXAMPLE 1 Panel specimens from the same lot of alloy 5252-H25 were buffed, cleaned in an inhibited alkaline cleaner, and bright dipped according to normal practice. The test specifications were anodized in a 15 percent sulfuric acid electrolyte at 25 C. at a current density of 12 a./s.f. to anodic oxide thicknesses of nominal 0.3 mil. The panels were then placed in an acidified solution of sodium sulfite at a pH of about 2 for approximately l5 minutes. The anodic coating after immersion had a weight loss of about 1.4 percent on the average.

EXAMPLE [I Panel specimens from the same lot of alloy 5252-H25 were submitted to the same buffing, cleaning and brightening steps as in example I and then anodized in an aqueous electrolyte containing 0.5 percent chromic acid and 10 percent sulfuric acid at the same temperature and current density to the same nominal 0.3 mil oxide thickness. The test specimens so anodized were then subjected to the same accelerated corrosion test as example I. The anodic coating showed a weight loss of about 0.6 percent on the average.

Other accelerated corrosion tests such as alternately immersing samples in a weakly acidic solution of copper chloride, sodium chloride and acetic acid and then air drying the samples, or subjecting the samples to a fog of the above solution (commonly termed the Cass test), showed the same improved corrosion resistance of oxide coatings produced in the 0.5 percent chormic acid-l0 percent sulfuric acid elec trolyte.

The aluminum alloys most benefited by the process of the invention are those alloys containing relative small amounts of alloying elements, such as those of the 100, 500 and some 6000 series Aluminum Association designated alloys. Total amounts of Mg, or Mg,Si are apparently not critical but the Mg, and Si should not exceed 1.5 percent and 1.0 percent respectively. Amounts, nominally in excess of 0.1 percent of elements such as copper, manganese, chromium and zinc, and iron in excess of 0.35 percent, do not allow the full benefit of the present invention to be obtained. Other impurities should not exceed 0.05 percent each.

It is obvious that various modifications of this invention can be made without departing from the spirit of the invention or the scope of the appended claims.

What is claimed is:

l. A method of fonning a highly durable, clear, colorless anodic oxide coating on an aluminum body comprising anodizing said body in an electrolyte consisting essentially of from 7 to 12 percent sulfuric acid, 0.25 to 0.8 percent chromic acid and the balance water, and during said anodization maintaining the current density between 10 and 30 a./s.f. in an electrolyte between 20 and 30 C.

2. The process of claim 1 wherein the aluminum body comprises an alloy consisting essentially of up to 1.5 percent magnesium, up to 1.0 percent silicon, not more than 0.10 percent each of copper, manganese, chromium and zinc, not more than 0.35 percent iron, not more than 0.05 percent each of other elements and the balance aluminum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3.616 ,310 Dated October 26 1971 In e t A Geoffrey A. Dorsey It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 48, "amounts of from" Sl'lOulfl be. amounts from 'Column 2, line 31, "Al 0" should be Al=O Column 2, line 35, "Al O" should be Al=0 3 Column 2, line '51, "cm should be cm" Column 2, line 59 "cm should be cm Column 3, line 21, "Al 0" should be Al=0 Column 3, line 41, "is was" should be it was Column 4, line 19, "Na Cr O" sliould be Na Cr O7 and Column 5, line 3, "100,500" shoull be 1'000,5o0o

Signed and sealed this 1 fih day of May 1972.

(SEAL) Attest:

EDWARD M.FLET( 3HER,JR. ROBERT GOTTSCHALK Attestlng Officer Commissioner of Patents 0-1050 10- 9 P 1 6 I USCOMM-DC scam-ps9 U 5 GDV ERNMENT PRINTING OFFICE: I965 O-366-334 

2. The process of claim 1 wherein the aluminum body comprises an alloy consisting essentially of up to 1.5 percent magnesium, up to 1.0 percent silicon, not more than 0.10 percent each of copper, manganese, chromium and zinc, not more than 0.35 percent iron, not more than 0.05 percent each of other elements and the balance aluminum. 